So-called reactive (thermosetting) hot-melt adhesives, which can be cured (crosslinked) after adhesion for improving properties such as heat resistance, are available as a kind of so-called hot-melt adhesives that can be used for thermal adhesion. The following adhesives 1) through 6) are exemplified as conventional reactive hot-melt adhesives:
1) Moisture-curing hot-melt adhesives comprising polymers having an isocyanate group (see U.S. Pat. No. 5,418,288 corresponding to JP-A-6-158017); PA1 2) Silanol-condensing hot-melt adhesives comprising polymers having a silyl group (see JP-A-5-320608, etc.); PA1 3) Radical-polymerization hot-melt adhesives comprising polymers having an acryloyl group (see JP-A-63-230781); PA1 4) Thermosetting hot-melt adhesives comprising polymers having a glycidyl group and phenol resins (see JP-A-617273 1); PA1 5) Hot-melt adhesives that are crosslinked by irradiation after thermal adhesion (see JP-A-6-306346); and PA1 6) Crosslinkable resin compositions comprising a terpolymer of ethylene, an .alpha., .beta.-unsaturated carboxylic acid, and an .alpha., .beta.-unsaturated carboxylate ester; a copolymer of ethylene and glycidyl methacrylate; and a diallyl phthalate compound as a crosslinking agent (see JP-A4-45123). PA1 a) Some hot-melt adhesives have a low crosslinking rate and require a long post cure time (for example, the above adhesives 1) and 2)); PA1 b) A certain hot-melt adhesive requires moisture for the crosslinking reaction, and is less suitable for adhering parts that are hardly in contact with air (for example, the above adhesive 1)); PA1 c) A certain hot-melt adhesive generates water as a byproduct, which has adverse affects, such as deterioration of adhesion strength with time (for example, the above adhesive 2)); PA1 d) Some hot-melt adhesives require a solvent for forming them in the form of a film and the residual solvent may have adverse effects after the completion of adhering (for example, the above adhesives 1) to 4)); PA1 e) In some hot-melt adhesives, the crosslinking reaction gradually proceeds when they are stored even at room temperature (about 25.degree. C.) and, therefore, they have low storage stability (for example, the above adhesives 1), 2) and 3)); PA1 f) Radiation-crosslinking adhesives are less suitable for adhering parts that cannot be or are hardly irradiated (for example, the above adhesive 5)); and PA1 g) Some adhesives (or compositions) require curing components such as curing agents that are contained in an originally active state. Thus, it is difficult to prevent gelation of such adhesives or compositions caused by heating during the step of processing them in a desired form such as a film and, therefore, they cannot be continuously produced (for example, the above adhesives 4) and 6)). PA1 i) a glycidyl (meth)acrylate monomer, and PA1 ii) an ethylene monomer. PA1 1. Copolymers of glycidyl (meth)acrylate and ethylene; PA1 2. Terpolymers of glycidyl (meth)acrylate, vinyl acetate and ethylene; and PA1 3. Terpolymers of glycidyl (meth)acrylate, ethylene and an alkyl (meth)acrylate.
Furthermore, U.S. Pat. Nos. 4,401,537; 5,709,948; and 5,721,289 disclose polymerizable compositions comprising an epoxy resin as an epoxy component, a cationic polymerization catalyst, and other components, that can be polymerized by polymerization procedures, including the irradiation of light such as ultraviolet (UV) rays, etc. Those compositions can be used as adhesives that are curable with light-irradiation because the cationic polymerization catalyst, which has been activated with light-irradiation, greatly increases the reaction rate of the epoxy resin. Examples of the other components are free radically polymerized polymers such as acrylic polymers that are disclosed in U.S. Pat. No. 5,721,289; thermoplastic polyolefins that are disclosed in U.S. Pat. No. 5,709,948; and the like. The above compositions can be used as curable adhesives (U.S. Pat. No. 5,721,289) and photocurable adhesives (U.S. Pat. No. 4,401,537), that have good reactivity.
Hot-melt adhesives containing polyethylene copolymers as adhesive components are desirably used in the electrical field. Such polyethylene type hot-melt adhesives are chemically stable, and their stability is proved by a test under severe conditions, for example, a pressure cooker test that is used to test semiconductor products. However, conventional polyethylene type hot-melt adhesives are not designed to perform a crosslinking reaction (curing reaction) between molecules of polyethylene copolymers. Thus, they have low heat resistance and cannot exert a sufficient bonding force at a temperature higher than a bonding temperature as can be readily understood from the adhesion mechanism of hot-melt adhesives. Such low heat resistance particularly limits the application of hot-melt adhesives in the electrical field. For example, when such adhesives are used as adhesion tapes to fix lead pins of IC lead frames, the tapes cannot be used in such conditions that the tapes are thermocompressed at a temperature of about 180.degree. C. and dipped in a solder bath, followed by maintaining them in a hot atmosphere of 230 to 260.degree. C.
The above conventional reactive hot-melt adhesives 1) through 6) still have one or more of the following drawbacks a) through g):
The above described epoxy resins are usually liquids at room temperature (around 25.degree. C.). Some epoxy resins are solids that can be liquefied by heating, but their flowability (for example, properties such as MFR, which will be explained later) under heated conditions is relatively large. Thus, an epoxy resin alone cannot be used as a hot-melt adhesive.
In addition, the high reactivity of epoxy resins makes it difficult to maintain both thermal curability and flowability while being heated of the compositions comprising the epoxy resins at high levels and to use the compositions as hot-melt adhesives. Furthermore, the epoxy resins tend to be separated from other components when the compositions comprising the epoxy resins are press adhered at a high temperature as hot-melt adhesives. Also, there is the fear of outgassing. The separated epoxy resins may lead to excessive flowability, low adhesion force, contamination of adherents, pollution of environment, and the like. When the epoxy resins are separated, the elasticity of the compositions cannot be sufficiently increased after the compositions are thermally cured.
Furthermore, the above resin composition comprising an epoxy resin and a cationic polymerization catalyst does not contain a polyethylene copolymer as an essential component and, therefore, has less chemical stability than the above polyethylene-type hot melt adhesives.
Thus, the object of the present invention is to provide a hot-melt adhesive composition, which can help solve the problems a) through g) associated with the above conventional reactive hot-melt adhesive compositions, the problems associated with the above compositions comprising the epoxy resin when used as the hot-melt adhesives, and also the problems associated with the conventional polyethylene type hot melt adhesives at the same time.